Visual transduction begins with the absorption of a photon by rhodopsin. Photoisomerization of the chromophore, from 11-cis-retinal to all-trans-retinal, must somehow signal the photoreceptor synapse to alter its release of transmitter onto second-order neurons. This signal is produced by the cyclic GMP-gated ion channel, a protein that regulates the voltage across the membrane in rods and cones. This ion channel, then, converts the signal of light into an electrical signal the brain can understand. This proposal focuses on the cGMP-gated ion channels, their role in phototransduction, their modulation by physiological stimuli, and the molecular mechanism by which cGMP binding leads to opening of the ion-conducting pore. We will examine the different components of the cGMP-gated ion channel protein to determine how they come together and how they interact. As a dynamic protein molecule whose structure and function change according to light levels, changes in interactions between parts of this protein are critical in allowing it to perform its job in vision. We will use the following tools to study how this protein works: electrical recordings, to assay the flux of charged ions through the pore of the protein; molecular biology to alter the amino acid sequence of the protein and test specific models of its function; and biochemical assays to study interactions between different parts of domains of the protein. Through these experiments we expect to gain significant insight into how the cGMP-gated ion channels function during visual transduction.